1. Field of the Invention
The present invention relates to a predictive coding device and, more particularly, to a coding device suitable for use in digital transmission of a voice, an image and so forth.
2. Description of the Prior Art
It has been proposed to provide various coding methods in order to reduce the amount of data transmitted during digital transmission of information such as image or voice information. Differential pulse-code modulation (hereinafter referred to as "DPCM") is one known example of such a coding method in which the amount of information is reduced by utilizing the correlations between adjacent sampled values.
As is well known, in DPCM, a coded sampled value is decoded, the decoded value is then used to obtain a predicted value for the succeeding sampled value to be coded, and then the error between the predicted value and the succeeding actual sampled value is quantized and coded.
FIG. 1 is a block diagram showing the construction of a DPCM coding device based on first-order prediction, which is the most simple. A sampled value X.sub.i is input through an input terminal 1 and applied to a subtractor 2, where a predicted value (in this example, a first-order decoded value) which will be described later is subtracted from the sampled value X.sub.i. A quantizer 3 quantizes the differential value output from the subtractor 2 and supplies the quantized differential to an output terminal 8 as a DPCM code Y.sub.i. Simultaneously, the DPCM code Y.sub.i is applied to an inverse quantizer 4. The inverse quantizer 4 decodes the DPCM code Y.sub.i into the differential value and applies it to an adder 5. In the adder 5, the first-order predicted value applied to the subtractor 2 is added to the differential value, and thus the original sampled value is recovered from the differential value. A limiter 6 limits the amplitude of the output of the adder 5 within a predetermined range and supplies the result to a D-type flip flop 7. The output of the D-type flip flop 7 is supplied to the subtractor 2 and the adder 5 as the predicted value for the succeeding sampled value.
In general, the distribution of differential values with respect to first-order predicted values is biased to a small-value portion. Accordingly, if the differential values are coded and transmitted, information can be transmitted in a compressed form.
However, in a case where signals of high sampling frequencies such as HDTV (high-definition television) signals are handled, even if a high-speed logic IC is used, it is impossible to complete, within one sampling period, the requisite arithmetic operations within a DPCM loop (the loop from the input of the sample value X.sub.i to the finding of a local decoded value which is the input to the D-type flip flop 7). For this reason, in order to handle such signals of high sampling frequencies, it is necessary to incorporate parallel processing. For example, for the case of a sampling frequency of 48.6 MHz, the sampling period is 20.5 ns. In this case, even if "FAST" which is a high-speed TTL-IC is used as a logic IC and even if a high-speed PROM is used as the quantizer 4, approximately 165 ns will be needed as the time interval for completing all the arithmetic operations within the, DPCM loop. Since 165/20.5=8.05, the number of parallel processings is nine, that is to say, nine identical circuits are required. As a matter of course, if even more complex processing is to be performed, the number of parallel processings most be increased even further. In addition, if the parallel processing is adopted, a circuit for dividing a signal into nine phases and a circuit for multiplexing the nine signals thus divided would also be required. As a result, the construction of the requisite hardware becomes complicated and the scale thereof extremely large.
Furthermore, if the quantizing characteristics of the quantizer are to be altered so that a coding device having the above-described arrangement may be adapted to various applications, the hardware construction of the quantizer itself must also be changed. Accordingly, it has heretofore been very difficult to change the quantization characteristics.